Patent GB-895353-A (GB)
Title: Density measuring apparatus and method
Inventor: ALCOCK NORMAN ZINKAN
IPC: G01N23/16
Filed: 1958-08-13
Abstract: 895,353. Beta-ray density gauges. CANADIAN-CURTISS-WRIGHT Ltd. Aug. 13, 1958, No. 25967/58. Class 40(3). In a system for measuring the mass per unit area of a material by passing beta rays through the material to an ionization chamber substantially fixed relative to the source, the part of the area of the chamber which is nearest the source is screened by a total absorber, and it is arranged that the angle of emission of the source, although large enough to cause radiation to fall outside the screen is not enough to illuminate the whole area of the chamber. Changes in detected radiation due to vibration or altered dimensions of mountings are thereby minimised. In Fig. 15 the source 147 emits beta rays over the angle A, and these pass through the material 149 under test and fall on the detector 146. The central area of the detector is screen at 150 and the angle A is insufficient for the rays to reach the edge of the exposed part at 151. The amount of radiation falling on the detector is then substantially independent of movement of the source or material, such as might occur when moving the - apparatus along or across a wide strip. Circuitry (Fig. 5). The chamber just described (D1) and a compensating detector (D2) are arranged in a bridge circuit whose output is amplified at 53 and indicated at 54, the sensitivity being controlled at 58. The manual control 50 provides for the cancellation of a steady reading, e.g. corresponding to density of the base when coating alone is to be measured, or to normal density of the coating when only variations are to be indicated. The adjusting potentiometer 50 is ganged with a variable resistor 48 which unbalances the bridge 45 and causes the servomotor 43 to adjust a diaphragm 40 between the compensating detector D2 and its source 39; at the same time the motor rebalances the bridge at 44 and displays the amount of compensation at 40b. Alternatively, switch 51 is thrown to the left so that the servomotor 43a rebalances the bridge and gives an indication at 40b. Construction of source-detector assembly (Fig. 6). The source is placed at 127 (detailed construction in Fig. 7, not shown) and the radiation therefrom can be turned off by the movable shutter 104 operated by the control 113. When the shutter is open, radiation passes upwards through the specimen to the ionization chamber 68 via the holes 65. The compensating detector D2 is located at 98 and the adjustable diaphragm 40 at 93, the radiation proceeding from the source 93a. The servomotor is at 88 and the rebalancing potentiometer 44 is at 90, the drive to the diaphragm 93 being via a worm shaft 91. Figs. 9-12 (not shown) illustrate the adjustment of the assembly to the specimen. Further compensation is included to allow for nuclear scattering of the beta rays by some elements (especially sulphur in rubber) which can upset calibration based on scattering by orbtial electrons only. This is partly compensated by absorbing the rays emitted in the angle B (Fig. 15) as described above, and also by including a "half-thickness" absorber 152 over the whole area of the detector. Alternatively (Fig. 16, not shown) this can be achieved by filling the chamber with a gas of high density, e.g. under pressure.